Fast closed-loop power control and soft handover (SHO), also referred to as macro diversity, are essential features of the 3rd-Generation Partnership Project (3GPP) standards for Wideband Code-Division Multiple (WCDMA) systems generally and of the Enhanced Uplink (EUL) features in particular. EUL is often referred to as High-Speed Uplink Packet Access (HSUPA), and is generally coupled with High-Speed Downlink Packet Access (HSDPA); these two high-speed packet data capabilities are collectively known as High-Speed Packet Access (HSPA).
In WCDMA systems, it is the Radio Network Controller (RNC) that is in control of reconfigurations, which implies rather long delays for performing a cell change. During SHO, the UE is power-controlled by the best uplink cell, i.e., by the cell that best receives the uplink transmissions from the UE.
FIG. 1 illustrates a traditional HSPA deployment scenario with two nodes having a similar transmit power level. Ideally, a mobile terminal (“user equipment,” or “UE,” in 3GPP terminology) moving from a serving cell towards a non-serving cell would enter the SHO region at point A. This is referred to as event 1A in 3GPP documentation. At point B, a serving cell change would occur, i.e., the non-serving cell becomes the serving cell and vice-versa. This is event 1D in 3GPP terms. At point C, the UE would leave the SHO region. 3GPP documentation refers to this as event 1B.
Since the nodes in FIG. 1 are assumed to have roughly the same transmit power, the optimal downlink (DL) and uplink (UL) cell borders will coincide, i.e., the path loss from the UE to the two nodes will be equal at point B. Hence, in an ideal setting and from a static (long-term fading) point of view, the serving cell will always correspond to the best uplink. However, in practice, due to imperfections in network control (e.g., reconfiguration delays) and fast fading, the UE might sometimes be power controlled by the non-serving cell during SHO. In such a case, there might be problems in receiving control channel information in the serving cell, due to the weaker link between the serving cell and UE. In some cases, it is essential that the uplink control channel transmissions are received by the serving cell. For example, the High-Speed Dedicated Physical Control Channel (HS-DPCCH) transmitted by the UE and scheduling information must be received in the serving cell.
Possible remedies to this problem include increasing the gain factors, by means of Radio Resource Control (RRC) signaling, or using repetitive transmissions, to improve the chances that the serving cell reliably receives the control channel information transmitted by the UE. A conventional remedy is simply to rely on retransmissions triggered by Hybrid Automatic-Repeat Request (HARQ) processes. However, while possible imbalances between the uplink and downlink in a traditional deployment are usually caused by fast fading, other factors can make the imbalance more pronounced in other scenarios, such as heterogeneous network deployments. In these scenarios, conventional retransmission techniques might not be adequate. Accordingly, improved techniques for handling uplink control channels in a SHO scenario are needed.